Shocking footage taken by an employee working undercover, shows the man Abbas, on the floor (Image: BBC)

He has tried to strangle himself after putting a mobile phone battery in his mouth (Image: BBC)

His colleague identified as Yan, comes in to help, as the detainee is pushing his fingers into his neck.

Yan shouts: "I'm going to put you to f****** sleep."

According to the report, Yan told Tulley not to file a Use of Force report, and claimed he 'wouldn't be bothered' if he had killed someone.

The Brook House facility holds up to 508 male detainees, all adults. The Home Office decides who is held there.

Managing Director for G4S Custodial and Detention Services in the UK, Jerry Petherick, said: “The behaviour shown in the programme is completely unacceptable and not representative of the many G4S colleagues who do a great job, often in difficult and challenging circumstances, across the country.

Callum Tulley, the undercover filmer, is helped by a member of staff called Yan, who is then heard shouting at the detainee (Image: BBC)

Yan was recorded saying 'I'm going to put you to f****** sleep' (Image: BBC)

"We have commenced an immediate investigation into the specific allegations made in the programme.

"I have re-emphasised to staff the importance of speaking out through whistleblowing channels if anyone is concerned about the behaviour of colleagues or standards at any of our facilities.

"We continue to focus on the care and wellbeing of detainees at Brook House."

A Home Office spokesman said: "We condemn any actions that put the safety or dignity of immigration removal centre detainees at risk.

"We are clear that all detainees should be treated with dignity and respect and we expect G4S to carry out a thorough investigation into these allegations and that all appropriate action be taken."

Brook House, near Gatwick Airport, where as many as 508 adult male detainees can be held at once (Image: PA)